As a fluid flows through a conduit or other restriction, static electrical charges are generated. If the fluid itself is not highly conductive, the charges do not leak away; but rather, there is a progressive build-up leading to significant charge densities. Because of the electrostatic field associated with these charges (either in the bulk or on surfaces) there may be a significant risk of fire or explosion if a spark should occur, especially if the fluid itself is combustible. Over the years, advanced designs have led to faster flow rates in fluid transfer systems and, as a result, the problems associated with electrostatic buildup have become more severe.
The dangers of electrostatic charge build-up during the pumping of hydrocarbon fuels are well-known, and the aviation industry has chosen to control such unwanted charge by the use of additives in the fuels. However, this solution is not practical for insulating fluids which are circulated in power system equipment, such as in power transformers The recent incidence of failure of large power transformers has generated the need for a simple, robust and reliable means to measure the charge density present in the oil circulating in a power transformer due to the streaming electrification process inherent in the pumping of the fluid, so that an alarm or corrective action may be initiated.
Another solution to this problem involved the monitoring of the electrostatic buildup until an upper level threshold was reached, at which point the charges could be dissipated using a charge neutralizer.
For example, an early method suspended a metallic sphere in the liquid. As the fluid passed by the sphere, the charged particles accumulated on the sphere; and a voltmeter measured the sphere's electrical potential. However, this method failed to give an accurate measurement because the fluid colliding with the sphere itself generated an excess charge which resulted in a false measurement.
If the fluid was pumped through an electrically isolated chamber, the relaxation of the charge in that chamber could be measured in terms of the current to ground. However, this cannot be interpreted as the charge density in the fluid unless the relaxation is completed, that is, the residence time in the chamber has to be large in comparison with the relaxation time of the fluid (which is largely determined by its conductivity). In some circumstances, this simple method may be used by providing suitable electrical isolation for the external transformer radiators (or other heat exchangers) so that they may be used as a relaxation chamber. However, this simple method has a disadvantage in that errors will accrue if relaxation in the chamber is not complete.
In U.S. Pat. No. 3,405,722 issued to Carruthers et al, a method was disclosed for making a more accurate charge density measurement by bleeding a small amount of fluid away from the primary conduit and into a bypass conduit having a fluid reservoir therein, and the electrostatic charge density of the fluid was measured by means of an electric field meter while the fluid remained in the reservoir. Thereafter, the fluid was returned to the primary conduit. However, this Carruthers et al '722 patent involved the use of a parallel diminutive simulation tank to mimic the behavior of the larger tank to be monitored, and the use of non-identical fluid paths involved inherent errors. A "scale factor" had to be empirically determined for the method to work, and there was no guarantee that this scale factor would be a constant, that is, independent of fluid properties, ambient conditions and flow regime. In particular, this scale factor was a function of fluid conductivity. Additionally, this system did not yield accurate measurements because electric field meters are inherently subject to interference.
U.S. Pat. No. 3,306,320 issued to Bond disclosed the use of an electrometer to measure the electrostatic phenomena in a branch line. However, this Bond '320 patent used the static electricity generating propensity of a fluid to detect changes in the fluid composition, and not for measuring the charge carried by the fluid itself. Moreover, this system required a means to periodically withdraw the fluid, rather than taking a continuous indication of charge; and the principle inherent in the Bond '320 patent involved the generation of a fluid charge and not the monitoring of the relaxation of the fluid.
In U.S. Pat. No. 4,309,661 issued to Kamoto, an electrically conductive segment of a bypass conduit absorbed the electrical charges carried by the fluid as the fluid traveled through the bypass conduit, and the charges were transferred to ground. An ammeter interrupted the ground path to provide a measurement of the electrical current generated by the charged fluid. A flow-rate measurement was also taken of the fluid in the bypass conduit, and the fluid charge density was calculated from current and flow rate using a well-known equation. However, to achieve an accurate measurement, the segment of electrically conductive bypass conduit had to be of sufficient length to permit the absorption of almost all of the electric charges in the electrified fluid. Meeting this constraint was a serious disadvantage and limitation, and contributed a source of error, especially in applications involving very high flow rates. Additionally, the bypass loop had a very small pressure drop between the entrance and exit ports to drive the flow, and the disclosure in Kamoto '661 did not employ a ram jet effect. Moreover, the entry region in Kamoto '661 was unscreened, and the charge migration to the walls contributed to errors in view of the low speed of bypass flow required to prevent charge segregation.
The following additional background patents are further illustrative of the prior art:
______________________________________ Inventor(s) U.S. Pat. No. ______________________________________ Polukhina et al 4,041,375 Owen 4,249,131 El-Menshawy et al 4,392,110 McHale et al 4,592,240. ______________________________________